The present invention relates to microrefrigerators and, in particular to a solid-state microrefrigerator based on normal metal-insulator-superconductor (NIS) tunnel junctions. The invention relates especially to a microrefrigerator using a single crystal as both the substrate and superconducting electrode of the NIS junction refrigerator.
NIS tunnel junctions are a promising technology for cooling to temperatures near 0.1 Kelvin (K) from bath temperatures near 0.3 K. These ultralow temperatures are desirable for the operation of thin-film sensors which measure energy deposited by particles and photons with great accuracy. Like any refrigerator, NIS junctions remove energy from one component, the normal metal electrode (a normal metal being any metal not in the superconducting state, e.g. silver, gold, copper) and dissipate a larger power in another component, the superconducting electrode. When a NIS junction is biased at a voltage V slightly below xcex94/e, where xcex94 is the energy gap of the superconductor, current flow through the junction preferentially removes the hottest electrons from its normal electrode. Refrigeration is therefore achieved in a solid-state device that operates without vibration or moving parts.
Cooling by NIS junctions was described by Nahum et al, Applied Physics Letters, 65 (24), 12 December 1994 (See also U.S. Pat. No. 5,634,718.) and development has since been pursued by two groups. The first group, at Harvard University, focused on junctions with dimensions of 10xc3x9710 microns or larger. They have produced the largest cooling powers to date, about 40 pW at 0.2 K, but only small reductions in temperature (See Fisher et al., Appl. Phys. Lett., Volume 74, Number 18, page 2705, May 3, 1999). The cause of this limited performance has been identified as heating in the superconducting electrode of these devices (See Ullom et al., Physica B 284-288 (2000) 2036-2038). The second group, at the University of Jyvxc3xa4skylxc3xa4 in Finland, has focused on devices with sub-micron dimensions (one micron or less) fabricated by electron-beam lithography (See U.S. Pat. No. 5,974,806). For devices of this size, heating of the superconducting electrode does not occur and the Jyvxc3xa4skylxc3xa4 work demonstrates that large temperature drops are feasible when this condition is met. Electrons have been cooled from 0.3 K to 0.1 K and photons from 0.3 K to 0.2 K (See Levio et al, AppL. Phys. Lett. 68,1996-1998 (1996) and Levio et al. Jun. 10, 1999). However, owing to the extremely small size of these devices, it is impossible to produce cooling powers much larger than 1 pW per junction at 0.3 K. As a result, these devices are probably only suited to cooling sub-micron sized hot electronic bolometers for millimeter wave measurements. To summarize, the Jyvxc3xa4skylxc3xa4 work demonstrates that if heating in the superconducting electrode can be overcome, substantial reductions in temperature are possible. The work at Harvard has shown that the techniques of Jyvxc3xa4skylxc3xa4 cannot be applied on larger scales for fundamental physical reasons.
If arrays of low temperature detectors are to be cooled, it is essential to provide devices in which the refrigeration junction and the cooling power are both large. It is therefore the purpose of the present invention to overcome the effects that have previously prevented cooling in large NIS junctions.
It is therefore an object of the present invention to provide a microrefrigerator for cooling to temperatures near 0.1 K from bath temperatures near 0.3 K.
It is another object of the present invention to provide such a microrefrigerator providing a large cooling power.
It is another object of the present invention to provide such a microrefrigerator having a large cooling power and also having a large (10s to 100s of microns) NIS junction.
It is a further object of the present invention to provide an NIS refrigerator capable of cooling multiple low temperature detectors or an array of detectors.
It is still another object of the invention to reduce the amount of power that returns from the superconducting electrode to the normal electrode in an NIS refrigerator.
It is still another object of the present invention to keep the density of quasiparticles small in the superconducting electrode in an NIS refrigerator.
Briefly, these and other objects are provided by the present invention in which an ultra-pure superconducting single crystal is both the substrate and the superconducting electrode of the NIS junction of the NIS refrigerator. The refrigerator consists of a large ultra-pure superconducting single crystal forming the superconducting electrode and the device substrate and a thin film normal metal layer on top of the superconducting crystal forming the normal electrode, separated by a thin insulating layer forming a tunnel barrier. The superconducting crystal can be either cut from bulk material or grown as a thick epitaxial film. The large single superconducting crystal allows quasiparticles created in the superconducting crystal due to electrons tunneling from the normal electrode to easily diffuse away from the NIS junction through the crystal to traps of normal metal. This prevents the quasiparticles from returning across the NIS junction. In comparison to conventional thin film NIS refrigerators, the invention provides orders of magnitude larger cooling power by using the large crystal as the superconducting electrode. The invention can cool sensors from 0.3 K to an operating temperature of 0.1 K or 0.05 K and therefore allow operation of a cryogenic photon sensor using a relatively simple pumped helium-3 refrigerator, or it can be used to extend the operation time below 0.1 K of adiabatic demagnetization refrigerators.
Other objects, advantages and features of the present invention will become apparent from the following description when considered in conjunction with the accompanying drawings wherein like or similar reference characters refer to similar elements in the several views.